Digital archives protect important data collections from failures by making multiple copies at other archives, so that there are always several good copies of a collection. In a cooperative replication network, sites "trade" space, so that each site contributes storage resources to the system and uses storage resources at other sites. Here, we examine bid trading: a mechanism where sites conduct auctions to determine who to trade with. A local site wishing to make a copy of a collection announces how much remote space is needed, and accepts bids for how much of its own space the local site must "pay" to acquire that remote space. We define a spectrum of trading scenarios, ranging from a network of archives and digital libraries that trust each other, to a scenario where sites do as they please, including breaking the rules. Then, we focus on developing techniques for the scenarios where sites trust each other, although we discuss issues that may arise if sites are greedy or malicious. We examine the best policies for determining when to call auctions and how much to bid, as well as the effects of "maverick" sites that behave differently than other sites. Simulations of auction and trading sessions indicate that bid trading can allow sites to achieve higher reliability than the alternative: a system where sites trade equal amounts of space without bidding.